quoted

On Tegra, the following use cases need specific IOVA mapping.
1. Few MMIO blocks need IOVA=PA mapping setup.

In that case, why would we enable the IOMMU for that one device; IOMMU
disabled means VA==PA, right? Perhaps isolation of the device so it can only
access certain PA ranges for security?

The device(H/W controller) need to access few special memory blocks(IOVA==PA)
and DRAM as well. If IOMMU is disabled, then it has to handle memory fragmentation,
 which defeats the purpose of IOMMU support.
There is also a case where frame buffer memory is passed from BootLoader to Kernel and
display H/W  continues to access it with IOMMU enabled. To support this, the one to one
mapping has to be setup before enabling IOMMU.

-KR